Schizophrenia is the most disabling psychiatric disorder, with a lifetime prevalence of about one-percent in the population (Bromet et al, 1974). Because schizophrenia usually appears early in life and is often chronic, the costs of the disorder are substantial. The disorder appears to be uniformly distributed worldwide, although pockets of high or low prevalence may exist (Docherty et al., 1996). Unemployment rates can reach 70%-80% in severe cases, and it is estimated that schizophrenic patients constitute 10% of the totally and permanently disabled. Homelessness and schizophrenia have been linked; it has been estimated that about one-third of homeless single adults suffer from severe mental illness, largely schizophrenia.
The essential features of schizophrenia consist of a mixture of characteristic signs and symptoms that have been present for a significant length of time during a 1-month period with some signs of the disorder persisting for at least 6 months (According to Diagnostic and Statistical Manual of Mental Disorder-IV, hereinafter DSM-IV). The symptoms involve multiple psychological processes, such as perception (hallucinations), ideation, reality testing (delusions), thought processes (loose associations) feeling (flatness, inappropriate affect), behavior (catatonia, disorganization), attention, concentration, motivation (avolition, impaired intention and planning) and judgment. No single symptom is pathognomonic of schizophrenia. These psychological and behavioral characteristics are associated with a variety of impairments in occupational and social functioning. The disorder is noted for great heterogeneity across individuals and variability within individuals over time. It is also associated with an increased incidence of suicide, which occurs in up to 10% of patients (Dinagman et al. 1986; Tsuang, 1978; McGlasban, 1988).
The characteristic symptoms of schizophrenia have often been conceptualized as falling into two broad categories—positive and negative (or deficit) symptoms—with a third category, disorganized, recently added. The positive symptoms include delusions and hallucinations. Disorganized symptoms include disorganized speech (Docherty et al., 1996), disorganized behavior and poor attention. Negative symptoms include restricted range and intensity of emotional expression (affective flattening), reduced thought and speech productivity (alogia), anhedonia, and decreased initiation of coal-directed behavior (avolition) (McGlashan et al, 1992).
According to DSM-IV, subtypes of schizophrenia are defined by the predominant symptoms at the time of the most recent evaluation and therefore may change over time. These subtypes include 1) paranoid type, in which preoccupation with delusions or auditory hallucinations is prominent; 2) disorganized type, in which disorganized speech and behavior and flat or inappropriate affect are prominent; 3) catatonic type, in which the characteristic motor symptoms are prominent; 4) undifferentiated type, which is a nonspecific category used when none of the other subtype features is prominent; and 5) residual type, in which there is an absence of prominent positive symptoms but continuing evidence of disturbance (e.g., negative symptoms or positive symptoms in an attenuated form) (Barbeau et al, 1995).
The etiology of schizophrenia is still unknown. Recent advances in neuroscience and psychopharmacology have suggested a wide array of competing mechanisms that may be involved in schizophrenia, including a deficit in one or more neurotransmitters (e.g., dopamine, serotonin, GABA and glutamate), their second messengers (Kaiya, 1992; Yao et al., 1996; Strunecka et al. 1999), neurodevelopmental defects in brain (Raedler et al., 1998), autoimmune mechanisms (Ganguli et al. 1993) and potential genetic risk factors (Allen et al. 2008). Dopamine involvement in schizophrenia is still attracting considerable attention despite the lack of direct evidence for abnormal dopaminergic function in the disorder. This is primarily based on the high correlation between the therapeutic efficacy of antipsychotic drugs and their potency as dopamine receptors blockers (Seeman, 1987), and the ability of dopamine agonists (such as bromocriptine and L-DOPA) to induce acute psychotic symptoms with marked resemblance to schizophrenia. It has been suggested that acute psychotic episodes are associated with a hyperdopaminergic state in the mesolimbic regions, while negative symptoms are associated with a hypodopaminergic state in the mesocortical projections to the frontal cortex (Davis et al., 1991). More recent, direct evidence for dopamine hyperactivity has emerged from imaging studies implicating dysfunction in dopamine metabolism, storage, release or uptake mechanisms in dopamine meso-limbic systems in schizophrenia (Breier A., et al., 1997. Laruelle et al., 1999).
Symptoms of other mental disorders, especially depression but also obsessive and compulsive symptoms, somatic concerns, dissociative symptoms and other mood or anxiety symptoms, are frequently seen with schizophrenia. The heterogeneity of the disorder and its comorbidity with symptoms of other mental disorders frequently renders schizophrenia difficult to diagnose. At present, definitive diagnosis of schizophrenia depends on descriptive behavioral and symptomatic information. Further, a very long time period, approximately 6 months, is required for the diagnosis of schizophrenia.
Since there is neither an effective biological marker for identifying schizophrenia (Willner, 1997; Hietala, et al., 1996), nor an accurate and rapid diagnosis for more optimal management of the disease at its different stages (Sheitman et al., 197) there remains an essential need for a reliable biological assay for the diagnosis and follow-up of schizophrenia. Identifying a peripheral biological marker will provide a) a more precise diagnosis and prognosis and might even shorten the 6-month period needed for diagnosis of schizophrenia; b) the possibility of using the peripheral marker as an objective tool for the patient's compliance to medication, if the marker responds differently in the presence of different types of antipsychotic drugs; and c) a correlation between the candidate marker and any feature of schizophrenia will contribute to the knowledge of the basic pathophysiology of the disorder, which might lead to new therapeutic strategies more specific and with fewer side effects.